A defect inspection of a micropattern formed in a semiconductor device manufacturing process such as a lithography process or etching process is described as an example of a conventional pattern defect inspection.
In response to recent increasing miniaturization of semiconductor patterns, defects that affect the performance of a semiconductor product have been decreasing in size. It has become increasingly difficult to defect such small-sized defects by conventional inspection apparatuses.
An inspection apparatus that has hitherto been in wide use acquires optical images or SEM images, and compares these inspection images with a reference pattern to defect defects. There are principles used for general defect inspection apparatuses; for example, a die-to-die method that uses, as a reference pattern, a die adjacent to an inspection pattern, and die-to-database method that uses design data as a reference pattern.
These inspection techniques vary in advantages and disadvantages depending on whether the inspection images are optical images or SEM images. The inspection based on the optical images allows a high throughput, but has the disadvantage of insufficient resolution for the detection of micro defects. On the other hand, the inspection based on the SEM images provides sufficient resolution but a low throughput. Therefore, the inspection based on the SEM images is not practically used for the inspection of the entire surface of a wafer at present, and is generally used for the inspection of an area that accounts for several percent of the entire wafer surface.
However, in order to predict an accurate product yield, it is desirable to increase the inspection area of a wafer to several ten percent which is equal to that in the optical inspection. Accordingly, various attempts have been made to increase the speed of the inspection based on the SEM images. While the SEM images are acquired by raster scans with an electron beam, the problem is that simply reducing the image acquiring time does not allow for an S/N of an image sufficient for a reliable inspection.
In the meantime, the inspection based on the SEM images allows the acquiring time of the SEM image to be reduced if an electron beam current amount is increased. However, the problem is that a charging phenomenon is induced and a desired image is not obtained when the electron beam current amount is simply increased. There has been suggested a technique that uses a mapping projection optical system to acquire the inspection images instead of using the raster scan for the image acquisition. However, this technique provides low image resolution and is disadvantageous to the detection of micro defects.